Many inorganic oxide particles are known in the art that are useful in sorbent applications, particularly as packing materials for High Performance Liquid Chromatography (HPLC).
The majority of separations employing high performance liquid chromatography are performed in the reversed-phase mode, wherein the column packing material surface is modified with a hydrophobic layer. In reversed-phase mode the column packing, or stationary phase, has is generally hydrophobic, while the mobile phase contains some fraction percent of water and therefore has more polarity. The most common presently used stationary phases employ a non-polar ligand (e.g., octane or octadecane) covalently bound to a porous silica (SiO2) particle through a siloxane bond to render the silica surface hydrophobic. Although these silica-based supports are very useful for a wide range of applications in reversed-phase HPLC, their use is typically limited to a pH range of between 2 and 8, due to the hydrolytic instability of both the silica support particle and the siloxane bond used to “anchor” the non-polar active group.
The strictly limited pH range for silica particles led, in part, to the use of other materials such as zirconia (ZrO2) as a substrate material for HPLC column packings. Particles made of zirconia have proven useful but have had other limitations. Some solutes, such as those that can act as a Lewis base, interact strongly with zirconia based particles and are not eluted with good peak shape or good efficiency. In extreme cases this interaction can result in irreversible adsorption of a solute on the chromatographic media. One approach to solving this problem has been the use of additives to the mobile phase capable of acting as a Lewis base, such as ammonium phosphate, sodium fluoride, sodium acetate or the like. However, this approach does not work with all applications (such as LC/MS which often requires volatile buffers) and still does not result in optimal chromatographic performance.
Therefore, a need exists for methods of modifying inorganic oxide particles so that Lewis base containing solutes can be eluted without the use of mobile phase additives and with enhanced chromatographic performance.